It is often necessary or desirable to monitor various parameters of blood and to obtain quantitative data concerning such parameters in real time. One way to accomplish this is to flow blood through a flow-through housing past sensors which provide signals representative of the parameters of interest. For example, Cooper U.S. Pat. No. 4,640,820 shows a flow-through housing with fluorescent sensors which respond to the partial pressure of oxygen, the partial pressure of carbon dioxide and the pH of blood which has passed through the flow-through housing.
Prior to using the flow-through housing, the sensors must be calibrated. One calibration technique, which is used for the sensors of the Cooper patent, is to attach the sensor carrier to a calibration housing containing calibration liquid. This places the sensors in communication with a relatively large cross-sectional area passage. The gas or gases of interest are then bubbled through the calibration liquid. A similar technique is utilized to calibrate the sensors shown in Maxwell U.S. Pat. No. 4,830,013.
Maxwell et al U.S. Pat. No. 5,057,278 discloses a calibration system in which sterile calibration liquid is equilibrated with the gas of interest and then pumped through the flow-through passage of the sensor cassette to be calibrated. Although this calibration system has many applications, it is particularly desirable for use with flow-through housings having a relatively small cross-sectional area.
In order to speed up the calibration process, it is desirable to reduce the time required for equilibration of the gas in the liquid. One way to accomplish this in a laboratory setting is to inject the gas through a porous, fritted stone into the liquid. This is accomplished in the laboratory by attaching a fritted stone to the end of a gas supply tube so that gas exiting from the tube must pass through the fritted stone. The gas supply tube with the fritted stone attached is then placed into a beaker containing the liquid. The gas then flows from the gas supply tube through the fritted stone into the liquid, and this does reduce somewhat the time required for equilibration. However, the fritted stone is relatively expensive and is not as efficient as desired in reducing equilibration time. Also, fritted stone is not suited for medical applications because of the possibility that it could introduce particulates into the liquid.
Calibration systems may employ tubing or other conduit means which are permeable to a component of air and/or the gas which is to be mixed with the calibration liquid. This permeability creates a possibility of air entering the calibration system or of the gas escaping from the system. In addition there is a possibility of mass transfer between various system components and the calibration liquid or between such systems components and the gas. These can be troublesome when it is necessary to accurately control the partial pressures of the gas of interest in the liquid.